Understanding the structure and properties of geological formations can reduce the cost of drilling wells for oil and gas exploration. Measurements made in a borehole (i.e., downhole measurements) are typically performed to attain this understanding, to identify the composition and distribution of material that surrounds the measurement device downhole. To obtain such measurements, gamma ray detectors are often used to measure naturally-occurring gamma radiation downhole. However, the output of some gamma ray detectors may fluctuate due to environmental conditions downhole. These fluctuations can cause changes in the apparent energy level detected by the gamma ray detector, leading to inaccuracies in the measurements reported by the tool. Thus, compensation techniques have been developed.
One is to implant an artificial radioactive source in the detector and use its signature energy as a reference. Another is to compare the signals from two detectors. However the first suffers from transportation restrictions, and is also limited by the computing power available downhole, or the available telemetry bandwidth, due to extensive calculations that implement whole spectrum curve fitting. The second increases the expense of the tool.